The use of switched mode output stage(s) in audio amplification, in which the conversion is performed digitally, allows audio output from digital signal input. The digital signal conversion may be achieved by a variety of devices. One particular technique utilizes a delta-sigma (ΔΣ) modulator, in which the digital input signal is filtered and then quantized to produce an output. In one switched technique, the noise-shaped quantized signal from the quantizer is coupled to a pulsewidth modulator (PWM), in which a value of the quantized signal controls the duty cycle of the PWM output. This PWM output then controls the operation of a switching device or devices, utilized as the output stage of an amplifier to drive a load. The filtered digital signal is used to modulate the pulsewidth of the PWM signal to control the duration of the switch on/off time of the output stage to the load. Thus, delta-sigma modulator circuitry coupled with a PWM circuitry may be utilized to operate as a switching amplifier to drive a load. Accordingly, digital signals are converted to an audio drive signal to drive a load, such as an audio speaker.
With the switching device(s) in the output circuitry of the switch mode amplifier, some amount of power is consumed. If the switching devices of the switching amplifier were ideal (that is, having zero impedance in the “on” state and infinite impedance in the “open” state) and switched instantaneously, the efficiency of the system would be at one hundred percent (100%). In the ideal case, all of the power provided by the power supply would be delivered to the load, and the switches would not dissipate heat. However, in actual practice, switches have some amount of heat loss, which, in some instances, may be significant. For example, the power loss may be in the order of ten percent (10%) of the input power. The switches are typically implemented using electronic devices, such as field-effect-transistors (FETs) or bipolar devices, although other devices may be utilized. These devices all have non-zero “on” resistance and require time to switch states, which may be significant in some instances. The effects typically will result in power dissipation in the switches, contributing to the heating of the switching devices.
Audio signals typically have a high peak power to average power ratio. In the long term, the average signal is significantly less than the maximum possible signal power. Since typical audio systems are designed to handle the average condition, many systems may nut be able to sustain a prolonged peak signal condition. The situation is especially true of consumer products, which are designed more for the average case and sacrifices ways to compensate for the worse case conditions. Generally, this approach is implemented to keep a lower cost for the system. Therefore, in some instances, when higher signal levels are processed, a tendency for the switches to overheat may exist and result in deterioration of the system response.
Generally, in order to prevent overheating of the switches that could result in component destruction, some switching amplifiers have employed protection mechanisms that open the circuit connection to the power supply or disconnect the load to prevent overheating or destruction of the switching circuit. For example, fuses may be employed to ensure an open circuit when the switching circuit draws excessive current. These known techniques cause an abrupt shut down of the system or abrupt disconnect of the signal to the output. Thus, the practical implication of these types of protection schemes results in the abrupt disconnect of the audio output to the speakers so that no sound emanates from the speakers.
A need and desire exist to have a less drastic way of addressing overheating conditions at the output instead of abruptly removing the audio output.